Knowledge of the flow rate of fluid in hydrocarbon well boreholes is important for monitoring and controlling fluid movement in the well and reservoir. Typically, in such hydrocarbon wells, the fluid flowing along the borehole includes a hydrocarbon (e.g. oil) and water. Each zone of the reservoir may have a valve to control the fluid inlet from that zone. By monitoring the flow rates of oil and water coming from each zone, the flow rate of oil can be optimised by control of the valves. In this way, the water cut may be minimised for example.
A flow meter installed in piping in a hydrocarbon well borehole should be able to operate relatively maintenance-free, due to the remoteness of its location. The flow meter should be able to cope with non-mixed and mixed flow regimes over a wide range of total flow rate and cut. Furthermore, the flow meter should not be sensitive to its orientation.
In non-mixed flow (such as stratified flow) the oil phase velocity and the water velocity are usually different. The difference in velocity is the phase slip. The volume fraction of water (holdup) must be measured in order to determine the flow rate of water, and similarly for the flow rate of oil. Thus, the determination of flow rate for a two-phase flow can be significantly more complex than for a single phase flow, and usually attempts are made to mix the flow before flow rate measurements are taken.
It is also often necessary to monitor or analyse the formation rock and the condition of piping and other components within the borehole. Measurements for this determination are usually taken by passing data logging tools along the pipeline. Such tools are dimensioned to pass along standard pipe sizes. Thus a flow meter for use downhole should be adapted to allow the unobstructed passage of logging tools.
It is known that flow rates can be measured on the basis of differential pressures in a flowing fluid. Differential pressure flow meters essentially fall into one of two types. The first type uses an inserted element of a design usually conforming to certain flow metering standards. The second type typically uses a piping element which dissipates energy from the flow and thereby leads to a pressure loss.
The first type of differential pressure flow meter includes devices which utilise e.g. orifice plates, Venturis, nozzles and Dall tubes, and rely on the conversion of pressure to kinetic energy in the fluid. In these flow meters, the equation for the differential pressure between two pressure measurement points (tappings) is given by                               Δ          ⁢                                           ⁢          P                =                  f          ⁢                                           ⁢          G          ⁢                                           ⁢          ɛ          ⁢                                           ⁢          ρ          ⁢                                    V              2                        2                                              (        1        )            
In Equation (1), f is a term related to the way the fluid moves through the device, G is a dimensionless term related to the geometry of the device, ε is the expansibility of the fluid (approximately unity for liquids), ρ is the density of the fluid, and V is the velocity of the fluid defined at some point in the device (e.g. at an inlet or throat). This is usually rearranged to the more familiar differential pressure flow meter equation:                               Q          m                =                  CE          ⁢                                           ⁢                      ɛ            ⁡                          (                              π                4                            )                                ⁢                      d            2                    ⁢                                    2              ⁢                                                           ⁢              ρ              ⁢                                                           ⁢              Δ              ⁢                                                           ⁢              P                                                          (        2        )            
In Equation (2), Qm is the mass flow rate, C is the discharge coefficient, E is a geometric term and d is the diameter of the throat or narrowest part of the flow.
Practical embodiments of Venturi-type flow meters for use in hydrocarbon well situations are given in WO00/68652.
The second type of differential pressure flow meter utilises the dissipative pressure drop created by elements of a piping system (such as elbows) to measure the flow rate. In devices of this second type, the pressure drop is given by an equation of the form shown in Equation (1) above with the various geometric and loss coefficients determined by experiments. For example, the pressure drop generated by an incompressible liquid flowing down a rough pipe is given by:                               Δ          ⁢                                           ⁢          P                =                  f          ⁢                      L            D                    ⁢          ρ          ⁢                                    V              2                        2                                              (        3        )            
In Equation (3), L is the length of the pipe and D is the diameter of the pipe. f is the friction coefficient and can be calculated from the Colebrook-White equation (D. S. Miller, “Internal flow systems” Chapter 8, BHRA, 1978):                     f        =                  0.25                                    [                                                log                  10                                ⁡                                  (                                                            k                                              3.7                        ⁢                        D                                                              +                                          5.74                                              Re                        0.9                                                                              )                                            ]                        2                                              (        4        )            In Equation (4) k is the pipe roughness (having the same units as D) and Re is the Reynolds number defined as:                     Re        =                              ρ            ⁢                                                   ⁢            VD                    η                                    (        5        )            where η is the viscosity of the flowing fluid.
Although geometrically similar devices might be expected to have the same friction coefficient, flow phenomena may occur which do not scale with Equation (3). Therefore, rather than applying a scaling equation, it is generally considered advisable to calibrate such meters separately.
A disadvantage of devices of both the first and second type is the sensitivity of such devices to the inlet flow conditions, such as swirl or pulsation. Also, particularly in respect of the second type, where the losses are due to sharp edges or surface roughness, corrosion and/or erosion can significantly affect the geometric and loss coefficients, and thereby impact on the accuracy of the predicted flow rates. Thus flow meters of this second type are usually only used where there is no possibility of incorporating “classical” flow meters of the first type such as Venturi flow meters.
For example flow meters of the second type may be used in very hostile or non-serviceable environments such as nuclear reactors, where the only feasible additions to the pipe work are pressure tappings for measurement of the differential pressure.
An alternative approach for measuring flow rate is based on the deliberate introduction of a swirl into the flow. The flow rate may then be determined by various means.
One option is to accelerate the swirl through a nozzle. Perturbations in the inlet flow (e.g. turbulent fluctuations) then become large amplitude pressure pulsations which can be detected and related to the flow rate. This principle is known as the “vortex whistle” and the rate of precession of the vortex is proportional to the flow rate.
A second option is to insert a vane or turbine in the swirling flow which is attached to a strain gauge or torque meter and is designed to detect only the force generated by the swirl which is again proportional to the flow rate.
In both cases, however, the “swirler” is very invasive to the flow. It is usually necessary to place a second flow conditioner downstream of the meter to straighten the flow.
Zarnett and Charles (G. D. Zarnett and M. E. Charles, Can. J. Chem. Eng., Vol. 47, (1969), 238-241) provide results from a laboratory study of air-liquid flows through a transparent plastic tube fitted with internal spiral ribs. The purpose of the study was to correlate flow patterns with pressure gradients. They did not suggest using the tube as a flow meter, and indeed such use would have been difficult to conceive of due to the difficulty of properly mixing the air-liquid flows.